<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS Sector</title>
  <style>
    .sector {
      position: relative;
      width: 100px;
      height: 100px;
      background-color: #3498db;
      border-radius: 50%;
      clip-path: inset(0 round 50% 50% 0 0);
      overflow: hidden;
    }

    .sector::before {
      content: '';
      position: absolute;
      top: 0;
      left: 0;
      width: 100px;
      height: 50px;
      background-color: inherit;
      border-radius: 100px 0 0 0;
    }
  </style>
</head>
<body>
<div class="sector"></div>
</body>
</html>
